In the microelectronic environment, there is a need for high density, high strength packaging to provide interconnection between semiconductor devices and connection from the devices to the electrical power supply. The electrical properties which are desirable include a highly conductive medium in a highly insulative carrier medium having a low dielectric constant. Thermally, the package must withstand not only the operating environment but also the thermal excursions encountered during the processing and fabrication of the part. Mechanically, it is preferable to have a substrate package which can withstand chip and pin joining stresses and stresses related to interconnecting with the next level of packaging. Furthermore, the package should be hermetic to prevent degradation of any of the desired properties due to leakage. Alumina has offered many of the mechanical properties needed, but does not have the optimum electrical and thermal characteristics. From an electrical standpoint, alumina has a relatively high dielectric constant. The thermal coefficient of expansion for alumina is not compatible with that of the silicon material used in chips. The thermal expansion incompatibility results in fatigue failures in the alumina-to-silicon interconnections. An additional drawback to the use of alumina is the fact that alumina has a very high sintering temperature (in the range of 1100.degree.-1600.degree. C.) which limits the choices for a co-sinterable metallurgy to high melting point refractory metals such as molybdenum or tungsten. The electrical properties of the high temperature metallurgies are less desirable; most notably, they have relatively high resistivity values.
Glass-ceramic composites have been explored as the dielectric materials to replace alumina. The compositions, having a low dielectric value and a favorable coefficient of thermal expansion, appear to be good alternatives to alumina. An added benefit to the use of composites is the lower temperature at which the materials sinter. Sintering temperatures in the range of 850.degree.-1000.degree. C. permit the use of cosinterable metallurgies such as gold, silver and copper, all of which have superior conductivity over the previously employed refractory metals. The disadvantage of using glass-ceramic compositions is that they generally have low strength and toughness as compared with the prior art dielectric materials. Low fracture strength results in cracking due to thermal expansion mismatch between the dielectric materials and the associated metallurgy. Should cracking occur during processing, the package may become permeable to processing solutions which could compromise the integrity of the package. A method has been taught in an accompanying pending application Ser. No. 167,606 filed concurrently with the subject application on Mar. 11, 1988, the teachings of which are hereby incorporated by reference, wherein a uniform gap is created between the ceramic and the associated internal metallurgy in order to provide an expansion zone between the thermally mismatched materials. Necessarily, the creation of such a gap results in exposure of the substrate to processing solvents which may directly affect the materials and less directly affect the performance of the part. The invention taught and claimed herein provides a solution to the hermeticity problem while simultaneously providing stress dissipation to prevent ceramic fracture.
Prior solutions to the stated problems include the use of dielectric materials to fill voids or cracks such as is taught in IBM Technical Disclosure Bulletin Volume 16, Number 2, page 624 (July 1973) and in U.S. Pat. No. 4,237,606. Filling voids and cracks with a glass layer, such as is taught in each of the references, can eliminate the porosity, provided the size of the pores and the surface tension of the material are not incompatible so as to prevent a complete fill. The problem of incomplete fill, mentioned above, is not an insignificant consideration. Filling the micropores encountered in ceramics and glass-ceramics processing requires materials which can fill spaces of submicron dimensions, which many of the non-reactive, inorganic dielectric materials cannot achieve. A further concern with the use of certain inorganic layers/fillers is that the fill material will not provide any relief from the stresses associated with the package and its use. Organic layers have been proposed as protective layers over completed modules wherein the devices have been mounted on the substrate and all interconnection is complete. In one example, IBM Technical Disclosure Bulletin Vol. 15, No. 6, page 1974 (November 1972), an article describes the use of an organic material layer to seal the assembly from the subsequently introduced cooling fluids. A similar use of an organic material deposited over a substrate assembly can be seen in IBM Technical Disclosure Bulletin Vol. 29, No. 3, page 1073 (August 1986) wherein a polymeric layer is used as a barrier to emission of alpha particles from the substrate. In each instance, the use of the organics is restricted to final processing, by which time many harmful fluids may already have been trapped in the assembly. Furthermore, planarity is not a concern in a finished assembly, as it is prior to device joining
It is therefore an objective of the subject invention to provide a method and structure for obtaining an hermetic substrate and for providing a planar low stress base for making interconnection to said substrate.
The subject invention not only deals with the sealing and stress difficulties but does so in situ at the substrate level thereby providing a superior, planar base for mounting devices.